Measure the size of the windows. Buy the acrylic sheets to double or triple the insulation. You can easily “stick” the acrylic sheets near your existing mineral glass. Use long magnets to close the doors. Old windows with double glazing will be easy to upgrade. Just open the inner window and “stick” the acrylic sheets 1,2 cm away from the existing mineral glass. Put one layer on your external window from the inside and one near the internal window, this way you will have 2 more layers. This way instead of 1 air “bubble” between 2 windows you’ll have 3 air bubbles. From the outside to the inside the layers will be the fallowing: existing widow, acrylic, acrylic, existing window.
Cheap and easy solution? Because the acrylic is on the inside of the existing windows, it’s quite safe to use it. Read about the negatives of the acrylic. With this method you will be able to achieve really good insulation even in historical old building. The best part of it is that you don’t have modify or change the facade or the interiors at all. It’s so simple to implement the idea that two 15 year old kids could do the whole job. They would be able to mount the acrylic sheets in their place within a few days.
They wouldn’t need any help from adults. In case of an error just grab the sheets and “glue” it back with the magnets. With this alone you could save a small fortune, especially with older houses. You don’t have to change the windows at all. This alone could save you literally hundreds of thousands in case of a big historic building.